Hello Carsten,
If you do find any information on the specific type of aircraft Bongartz flew I would be very grateful if you'd post it here. Given that a Kasta was usually composed of six aircraft I would have thought that of the links I posted it is far more likely that he flew a Rumpler C.I than any of the other types for which I posted a link.
I'm not sure if it's relevant to your needs, but just in case, Dirk Rottgardt in German Armies Establishments 1914/1918 vol.6 P.79 has Kagohl 5 and therefore Kasta 27 established in March 1916 rather than December 1915 as per
http://www.frontflieger.de/2-kasta27.html .
Also I don't have a copy but does Lance Bronnenkan't vol.14 of The Blue Max Airmen not mention the type of aircraft flown by Bongartz while with Kasta 27?
Regards,
Clint